1. Field of the Invention
The present invention relates to a cholesteric liquid crystal display used for a display panel in electronic equipment and a recording/display medium of images.
2. Discussion of the Related Art
The cholesteric liquid crystal display has attracted considerable attention in recent years as a display device for electronic paper such as electronic newspapers and electronic publications since it has the following advantages: being capable of utilizing reflection of surrounding lights to give a light display; having a storage property that holds display contents after the supply voltage is turned off; being capable of a large-capacity display by simple matrix drive utilizing the storage property; being capable of using a flexible substrate of a resin, etc., because an active matrix is not needed for the drive, and the like.
The cholesteric liquid crystal is made up of spirally oriented stick-like molecules, and exhibits a selective reflection phenomenon that reflects a light of a wavelength corresponding to a spiral pitch. The cholesteric liquid crystal display elements utilize this phenomenon. As an example of the sectional structure of this device is illustrated in FIG. 18, the device is made up of cells that sandwich a cholesteric liquid crystal 30 between two substrates 11, 12 each having interventional transparent electrodes 21, 22, and a light absorptive layer 41 that absorbs a selective reflection wavelength is attached on the opposite face to the cell observation side. The light absorptive layer 41 is presumed to be a black color hereunder.
The orientation of the cholesteric liquid crystal takes on three types, namely, planer (P) orientation, focal conic (F) orientation, and homeotropic (H) orientation, as shown in FIG. 17A through FIG. 17C. The P orientation is a state in which the spiral axis is oriented almost vertically to the substrate plane, which assumes a color according to a selective reflection wavelength. The F orientation is a state in which the spiral axis is oriented almost in parallel to the substrate plane, which is colorless, and the black color of the light absorptive layer 41 is observed. The H orientation is a state in which the spiral structure is decomposed and the molecules are oriented vertically to the substrate plane, which is also colorless, and the black color of the light absorptive layer 41 is observed.
When a voltage is applied across the transparent electrodes 21, 22, both the P orientation and the F orientation stably exist in the applied voltage lower than VT1, showing a bi-stable state. When the voltage is increased, the F orientation does not change and the P orientation transitions gradually into the F orientation; and when the voltage is over VT2, the state completely transitions into the F orientation. When a still higher voltage than VT3 is applied, the state is starting to transition into the H orientation, and when the voltage is over VT4, it completely transitions into the H orientation. Even though the applied voltage is sharply removed from the state of the F orientation, the F orientation is maintained; however, when the voltage is sharply removed from the state of the H orientation, it transitions into the P orientation.
As a result of the above transition characteristic, the measurement of the reflectance after a specific time from when the voltage is applied only for the time T as shown in FIG. 20 gives the voltage vs. reflectance characteristic as shown in FIG. 21. That is, when the initial orientation is the P orientation, the characteristic shows a high reflectance under VT1; in the range over VT1 under VT2, the reflectance gradually lowers; in the range over VT2 under VT3, the characteristic shows a low reflectance; in the range over VT3 under VT4, the reflectance increases; and over VT4, it shows a high reflectance, which is the same as that in the initial orientation. On the other hand, when the initial orientation is the F orientation, the characteristic shows a low reflectance under VT3; in the range over VT3 under VT4, the reflectance increases; and over VT4, it shows a high reflectance.
The above voltage vs. reflectance characteristic varies depending upon the time T during which the voltage is applied. When the initial orientation is the P orientation, as shown in FIG. 23, the whole voltage vs. reflectance characteristic shifts to a higher voltage side as the time T becomes shorter, and in the range over VT2 under VT3, the reflectance increases. This is because the transition into the F orientation becomes incomplete by the time T becoming shorter to create a state in which the F orientation and the P orientation are microscopically mixed. On the other hand, when the initial orientation is the F orientation, as shown in FIG. 22, VT4 shifts to a higher voltage side as the time T becomes shorter, and the range over VT3 under VT4 expands.
Utilizing the above voltage vs. reflectance characteristic, the cholesteric liquid crystal display is able to write image data by means of the simple matrix electrodes serving intersection portions of scan-electrodes and data-electrodes as pixels. As an example, FIG. 19 illustrates a plan configuration of a simple matrix panel having 16xc3x9716 pixels. As shown in the drawing, the panel contains a scan-electrode group 23 made up of R1 to R16 and a data-electrode group 24 made up of C1 to C16.
As a method of driving the cholesteric liquid crystal display elements, for example, the write method named as the FCR (ForcalConic Reset) method is disclosed in the Japanese Published Unexamined Patent Application No. Hei 11-326871. This method executes writing by a drive voltage made up of a reset time for making the pixels transition into the F orientation and a selection time for writing the P orientation, in which a drive voltage to make the pixels simultaneously transition into the F orientation is applied to all the scan-electrodes during the reset time, and next a selected voltage is applied to the scan-electrodes one by one sequentially.
FIG. 9 illustrates a timing chart of the drive voltage that is applied to the scan-electrode group 23 having 16 electrodes as an example. As in the drawing, during the reset time Tr, the method gives a voltage Vrh over VT4 to make the pixels transition into the H orientation, thereafter brings the voltage once to zero, next gives a voltage Vrf being over VT2 under VT3 and again brings to zero to thereby attain the F orientation. During that time, the voltage given to the data-electrode group 24 is zero. During the selection time Ts, the method gives the voltage Vs of (VT3+VT4)/2 to the scan-electrodes, and simultaneously gives a data voltage of (VT3xe2x88x92VT4)/2 or (xe2x88x92VT3+VT4)/2 to the data-electrodes. Thereby, VT4 or VT3 being the difference of the scan-voltage and the data voltage is applied to the pixels, which makes the pixels selectively transition into the P orientation or the F orientation. The voltage applied to the scan-electrodes is zero except the reset time Tr and the selection time Ts.
While selecting a scan-electrode, the method applies the data voltage (VT3xe2x88x92VT4)/2 or (xe2x88x92VT3+VT4)/2 to a pixel on another scan-electrode. To condition the data voltage as |(VT3xe2x88x92VT4)/2| less than VT1 will permit writing the data in all the pixels without varying the reflectance of the already written pixels. Assuming that the number of the scanning lines is N, the full write time Tf is given by the following expression 1.
Tf=Tr+Nxc3x97Tsxe2x80x83xe2x80x83[Expression 1]
Another method is disclosed in the specification of the U.S. Pat. No. 5,748,277, which is named as the DDS (Dynamic Drive Scheme) method. The DDS method takes on the drive voltage waveform, which is made up a series of reset time Tr, selection time Ts, and hold time Th, as shown in FIG. 24. During the reset time Tr, a voltage Vrh is applied to make the pixels transition into the H orientation. During the selection time Ts, a voltage Vs is applied to select maintaining the H orientation or starting transition into the P orientation. During the hold time Th, a voltage Vh is applied to maintain the H orientation and to make the P orientation transition into the F orientation. When the voltage Vs is selected so as to maintain the H orientation, after removing the hold voltage Vh, the liquid crystal transitions into the P orientation to assume a high reflectance. On the other hand, when the voltage Vs is selected so as to start transition into the P orientation, the liquid crystal transitions into the F orientation to assume a low reflectance. FIG. 25 illustrates the voltage vs. reflectance characteristic with regard to the voltage Vh, in Vs=0 and Vs=Vh. In Vs=0, the characteristic becomes equal to that in case of the initial orientation being the P orientation. In Vs=Vh, the characteristic assumes a shape such that the characteristic in Vs=0 is shifted to a lower voltage side. The voltage Vh is selected to be over VT5 under VT3. The voltage vs. reflectance characteristic with regard to the voltage Vs is as shown in FIG. 26, and the reflectance can be controlled within the range over VT6 under VT7.
This drive method can be applied to a simple matrix panel. FIG. 10 illustrates a timing chart of the drive voltage that is applied to the scan-electrode group 23 having 16 electrodes as an example. The method applies a drive voltage Vrh, Vs, Vh corresponding to the reset time Tr, selection time Ts, hold time Th to the scan-electrodes sequentially with a shifted timing of the selection time length Ts. During the selection time Ts, the method gives the voltage (VT6+VT7)/2 to the scan-electrodes, and to synchronize with it, gives the voltage (VT6xe2x88x92VT7)/2 or xe2x88x92(VT6xe2x88x92VT7)/2 to the data-electrodes. Thereby, VT6 or VT7 being the difference of the scan-voltage and the data voltage is applied to the pixels, which makes the pixels selectively transition into the P orientation or the F orientation. To set the data voltage as |(VT6xe2x88x92VT7)/2| less than VT1 will permit writing the data in all the pixels without varying the reflectance of the already written pixels. The full write time Tf is given by the following expression 2.
Tf=Tr+Nxc3x97Ts+Thxe2x80x83xe2x80x83[Expression 2]
Both the FCR method and the DDS method utilize the storage property of the cholesteric liquid crystal, and write in the pixels on the next scan-electrodes without varying the reflectance of the already written pixels. Thereby, the both methods allow a large-capacity display that does not limit the number of the scan-electrodes.
However, both the FCR method and the DDS method inevitably increase the full write time Tf, as the number of the scan-electrodes increases. That is, the second term in the [expressing 1] and [expression 2], Nxc3x97Ts, makes a dominant contribution. Normally, the length of the selection time Ts is 1 to 10 ms/line in the FCR method, and 0.3 to some ms/line in the DDS method, although it cannot be specified without reservations, since it depends on the physical constant, cell parameter, applied voltage, and so forth. When the number of the scanning lines is 1000, for example, the rewrite time is 1 to 10 sec in the FCR method, and 0.3 to some sec in the DDS method. In a low temperature, it takes time of several times more, due to the rise of the viscosity of a liquid crystal. This rewrite time cannot necessarily be said sufficiently short depending on the applications, and a still more shortening of the rewrite time has been desired.
In case of the FCR method, the length of the selection time Ts depends on the viscosity, orientation elasticity constant, and dielectric anisotropy, etc., of a liquid crystal, however there have been limits to improvements by these physical constants. Further, as shown in FIG. 22, a rise of the drive voltage will shorten the selection time, however it will cause a cost increase in the drive circuit, a yield decrease by short-circuits between the electrodes, and an increase in the power consumption. Further, shortening of the selection time by increasing the drive voltage will effect the applied voltage |(VT3xe2x88x92VT4)/2| to the data-electrodes to exceed VT1, leading to creating crosstalks, which is a problem. In case of the DDS method, the length of the selection time is determined by the physical constants of the viscosity and orientation elasticity constant of a liquid crystal and the like, however there have been limits to a shortening of the selection time by these.
The present invention has been made in view of the above circumstances, and provides a cholesteric liquid crystal display capable of rewriting at a high speed.
According to one aspect of the invention, the cholesteric liquid crystal display includes cholesteric liquid crystal display elements forming pixels at intersection portions of a scan-electrode group and a data-electrode group; and a drive circuit that sequentially selects scan-electrodes of the scan-electrode group as a block made up of plural scan-electrodes, simultaneously applies coded drive voltages each corresponding to the plural scan-electrodes in the block in a selection time, and applies coded data-voltages each corresponding to data-electrodes of the data-electrode group synchronously with the drive voltages.
Here, as the drive voltages to be used may be those that have, during a time over 50% of the selection time, a peak value equal to or higher than a voltage that makes the pixels transition into the homeotropic orientation, and are coded by means of an orthogonal function or a substantially orthogonal function. The orthogonal function to be used may be one that takes +1 and xe2x88x921 as elements, however it is not limited to this. Further, the data-voltages may take on those that are coded by multiplying an orthogonal function or a substantially orthogonal function by pixel data values.
The selection time can be made to include plural orthogonal cycles that represent a time for satisfying an orthogonal condition of the orthogonal function. The response time of a liquid crystal to an effective voltage that is applied to the pixels within the selection time is equal to or longer than one the orthogonal cycles, and equal to or shorter than the selection time. Further, the effective voltage applied to the pixels within a non-selection time is smaller than a threshold voltage that allows maintenance of a bi-stable state with the planer orientation and a focal conic orientation.
Further, a reset time for making the pixels transition into an initial orientation may be provided before the selection time. The reset time is given simultaneously to all the blocks, or it is given to each block sequentially with a shifted timing. Further, a hold time for supporting a transition into a final orientation state may be provided after the selection time. The block is made up of plural spatially separated scan-electrodes, not of plural adjoining scan-electrodes.
According to another aspect of the invention, the image-writing device that writes images in cholesteric liquid crystal display elements forming pixels at intersection portions of a scan-electrode group and a data-electrode group includes an orthogonal function generating circuit that generates an orthogonal function; a scan-voltage composing circuit that generates scan-voltages by level-shifting the orthogonal function, the scan-voltages are sequentially applied to every plural scan-electrodes of the scan-electrode group; and a data-voltage composing circuit that generates data-voltages by level-shifting a value of multiplying the orthogonal function by a pixel data value, the data-voltages are applied to data-electrodes of the data-electrode group. To this image-writing device may be provided a scan-electrode driver capable of applying the scan-voltages to every plural spatially separated scan-electrodes of the scan-electrode group. The device may also be provided with a reset waveform generating circuit that applies a reset waveform through the scan-voltage composing circuit and the data-voltage composing circuit, before applying the scan-voltages. Further, applying a waveform having an arbitrary phase shift as the pixel data value will make it possible to display the gradations.
According to another aspect of the invention, the method of writing images in cholesteric liquid crystal display elements forming pixels at intersection portions of a scan-electrode group and a data-electrode group includes the steps of selecting sequentially scan-electrodes of the scan-electrode group as a block made up of plural scan-electrodes, applying simultaneously coded drive voltages each corresponding to the plural scan-electrodes in the selected block, and applying coded data-voltages each corresponding to data-electrodes of the data-electrode group synchronously with the drive voltages. Here, the drive voltages may be attained by level-shifting the orthogonal function that takes +1 and xe2x88x921 as elements.
With the above construction, the present invention achieves simultaneous writing of images in L-lines of the scan-electrodes (L: integer larger than 2). Thereby, the length of the selection time can be shortened to substantially 1/L at maximum. Therefore, the rewrite time can be reduced as a whole, which makes it possible to provide a cholesteric liquid crystal display capable of rewriting at a high speed.